DFT calculations of the structure and stability of copper clusters on MoS₂

Abstract

Layered materials, such as MoS2, are being intensely studied due to their interesting properties and wide variety of potential applications. These materials are also interesting as supports for low-dimensional metals for catalysis, while recent work has shown increased interest in using 2D materials in the electronics industry as a Cu diffusion barrier in semiconductor device interconnects. The interaction between different metal structures and MoS2 monolayers is therefore of significant importance and first-principles simulations can probe aspects of this interaction not easily accessible to experiment. Previous theoretical studies have focused particularly on the adsorption of a range of metallic elements, including first-row transition metals, as well as Ag and Au. However, most studies have examined single-atom adsorption or adsorbed nanoparticles of noble metals. This means there is a knowledge gap in terms of thin film nucleation on 2D materials. To begin addressing this issue, we present in this paper a first-principles density functional theory (DFT) study of the adsorption of small Cu n (n = 1–4) structures on 2D MoS2 as a model system. We find on a perfect MoS2 monolayer that a single Cu atom prefers an adsorption site above the Mo atom. With increasing nanocluster size the nanocluster binds more strongly when Cu atoms adsorb atop the S atoms. Stability is driven by the number of Cu–Cu interactions and the distance between adsorption sites, with no obvious preference towards 2D or 3D structures. The introduction of a single S vacancy in the monolayer enhances the copper binding energy, although some Cu n nanoclusters are actually unstable. The effect of the vacancy is localised around the vacancy site. Finally, on both the pristine and the defective MoS2 monolayer, the density-of-states analysis shows that the adsorption of Cu introduces new electronic states as a result of partial Cu oxidation, but the metallic character of Cu nanoclusters is preserved.

Prediction of Co and Ru Nanocluster Morphology on 2D MoS2 from Interaction Energies

“…As in our previous work, [28], there are three metal atom adsorption sites labelled as 1, 2 and 3 on the MoS 2 ML, which are highlighted in Figure 1A. Site 1 has a metal atom adsorbed directly atop an S atom.…”

“…The description of the pristine and defective MoS 2 monolayer (ML) was published in our previous work. [28] Bulk MoS 2 is made up of two layers. To create the pristine ML one of these was removed, which also creates the vacuum necessary to avoid interaction along the z-axis; the vacuum region is 8 Å.…”

“…Co and Ru atoms adsorb exothermically at all three adsorption sites. A single Co atom adsorbs most strongly at site 2, with an energy gain of -5.82 eV, followed by site 3 and site 1, with energy gains of - [28]; the presence of these could be one origin for the enhanced interaction between Co and the MoS 2 ML. The relaxed geometries for adsorption of Co 1 and Ru 1 are shown in Figures 1 and 2, respectively.…”

“…In our previous study we identified that while these studies do deliver useful insights, there is a knowledge gap in the understanding of metal thin film nucleation on 2D materials. [28] We showed that we can investigate the first stages of thin film nucleation on 2D materials with first principles simulations, using the example of small Cu n structures on an MoS 2 monolayer (ML).…”

“…[29,11]. Our previous first principles study [28] of the interaction of Cu species on MoS 2 showed how Cu can take different structures depending on the number of Cu atoms and whether the TMD is stoichiometric or defective.…”